Measurement and correction of optical aberrations in charged particle beam microscopy

Abstract

To suppress optical aberrations in the microscope that can limit resolution of images, as much as possible.SOLUTION: A charged particle beam microscope system is operated in a transmission imaging mode. During the operation, the charged particle beam microsystem directs a charged particle beam to a sample to produce images. A time series of beam tilts is applied in a pattern to the charged particle beam directed to the sample to produce a sequence of images. At least some of the images in the sequence of images are captured while the charged particle beam is transitioning between one beam tilt in the time series of beam tilts and a sequentially adjacent beam tilt in the time series of beam tilts. The pattern is configured to induce image changes between the images in the sequence of images that are indicative of optical aberrations in the charged particle beam microscope system.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The field relates to charged particle beam microscopy. [Background technology]

[0002] Transmission electron microscopy TEM (transmission electron microscopy) is a technique in which a beam of electrons is transmitted through a thin sample to form an image. The image is formed by the interaction of the electrons with the atoms of the sample as they pass through the sample. The image can be captured by a camera. TEM can produce high-resolution images due to the short wavelength of the transmitted electrons. Images with a sufficiently high resolution can be used to study and analyze details of the sample's structure on an atomic scale. However, optical aberrations in the microscope can limit the resolution of the image. [Brief description of the drawings]

[0003] [Figure 1] FIG. 1 is a block diagram of a system for measuring and correcting optical aberrations in a charged particle beam microscope system operating in transmission imaging mode according to various implementations. [Figure 2A] FIG. 1 is a flow diagram of a method for measuring and correcting optical aberrations in a charged particle beam microscope system operating in transmission imaging mode, according to various implementations. [Figure 2B] FIG. 1 is a flow diagram of a method for measuring and correcting optical aberrations in a charged particle beam microscope system operating in transmission imaging mode, according to various implementations. [Figure 3A] 1 is a graph of a Lissajous figure beam tilt pattern. [Figure 3B] 3B is a graph of the expected induced image shift pattern produced by the Lissajous figure beam tilt pattern depicted in FIG. 3A when there is defocus in a charged particle beam microscope system. [Figure 3C]3B is a graph of the expected induced image shift pattern produced by the Lissajous figure beam tilt pattern depicted in FIG. 3A when there is defocus and astigmatism in a charged particle beam microscope system. [Figure 3D] 3B is a graph of the expected induced image shift pattern produced by the Lissajous figure beam tilt pattern depicted in FIG. 3A when a charged particle beam microscope system has defocus, coma, and astigmatism. [Figure 3E] 3B is a graph of the expected induced image shift pattern produced by the Lissajous figure beam tilt pattern depicted in FIG. 3A when a charged particle beam microscope system has defocus, coma, astigmatism, and spherical aberrations. [Figure 4A] 1 is a graph of a circular beam tilt pattern. [Figure 4B] 4B is a graph of the expected induced image shift produced by the circular pattern depicted in FIG. 4A when there is defocus in a charged particle beam microscope system. [Figure 4C] 4B is a graph of the expected induced image shift produced by the circular pattern depicted in FIG. 4A when there is defocus and astigmatism in a charged particle beam microscope system. [Figure 4D] 4B is a graph of the predicted induced image shift produced by the circular pattern depicted in FIG. 4A when there is defocus, astigmatism, and coma in a charged particle beam microscope system. [Figure 5A] FIG. 1 is a flow diagram of an exemplary method of operating a charged particle beam microscope system with aberration monitoring and correction. [Figure 5B] FIG. 11 is a flow diagram of another exemplary method of operating a charged particle beam microscope system with aberration monitoring and correction. [Figure 6] FIG. 11 is a flow diagram of another exemplary method of operating a charged particle beam microscope system with aberration monitoring and correction. [Figure 7]FIG. 1 is a generalized computing system block diagram of a suitable computing environment that may be used in a method for implementing monitoring and correction of optical aberrations in a charged particle beam microscope system. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0004] The subject matter disclosed herein relates to methods and systems for measuring and correcting optical aberrations, for example in a charged particle beam microscope system operating in a transmission imaging mode. Examples of optical aberrations that may be measured and corrected include, but are not limited to, defocus, 2-fold astigmatism, 3-fold astigmatism, 4-fold astigmatism, 5-fold astigmatism, 6-fold astigmatism, axial coma, 5th order axial coma, spherical aberration, 6th order spherical aberration, star aberration, 6th order star aberration, 3-lobe aberration, and rosette aberration.

[0005] In one or more implementations, the system includes a charged particle source that emits a charged particle beam, an optical system including one or more optical components configured to define an optical axis and form the charged particle beam within a field of view of a sample positioned along the optical axis, a pattern generator configured to generate a pattern, one or more beam deflectors positioned along the optical axis and controllable by the pattern to apply a time series of beam tilts to the charged particle beam, and an imaging sensor positioned along the optical axis to capture an image of the sample.

[0006] In one or more implementations, while operating the charged particle beam microscope system in a transmission imaging mode and directing the charged particle beam to a sample, a time sequence of beam tilts is applied to the charged particle beam in a pattern to produce a series of images. At least some of the images in the series of images are captured while the charged particle beam transitions between one beam tilt in the time sequence and a sequentially adjacent beam tilt in the time sequence. In one or more examples, the series of images can be analyzed to determine optical aberrations in the microscope system.

[0007] In one or more examples, the pattern is configured such that the beam tilt induces an image change (e.g., an image shift) in the series of images that can be correlated to optical aberrations in the microscope system. In one or more examples, the image change can be determined from the series of images and used to estimate one or more optical aberration values ​​of the microscope system. In other examples, the one or more optical aberration values ​​can be determined by a trained neural network that receives as input data the series of images or information derived from the series of images (e.g., the power spectrum of the images).

[0008] In one or more examples, the beam tilt pattern can be a periodic pattern, such as a Lissajous figure. The use of a periodic pattern can allow unknown time delays and unknown deviations in the frequency of the applied beam tilt pattern to be determined in relation to the measured image change over time. Periodic patterns can also allow the system to be robust to other effects that can induce image changes (e.g., sample drift and mechanical vibrations) and allow signal processing to be used to separate different signal sources.

[0009] The subject matter is described with implementations and examples. In some cases, as will be recognized by those skilled in the art, the disclosed implementations and examples can be implemented without one or more of the specific details disclosed, or can be practiced with other methods, structures, and materials not specifically disclosed herein. All implementations and examples described herein and shown in the drawings can be combined without any restriction to form any number of combinations, unless the context clearly dictates otherwise, such as when the proposed combination involves elements that are incompatible or mutually exclusive. The sequential order of operations in any process described herein can be rearranged, unless the context clearly dictates otherwise, such as when one operation requires the result of another operation as input.

[0010] For brevity and continuity of description, the same or similar reference numbers may be used for the same or similar elements in different figures, and the description of an element in one figure is considered to be carried forward when that element appears in another figure with the same or similar reference number. In some cases, the term "corresponding to" may be used to describe the correspondence between elements in different figures. In exemplary usage, when an element in a first figure is described as corresponding to another element in a second figure, the element in the first figure is considered to have the characteristics of the other element in the second figure, and vice versa, unless otherwise noted.

[0011] The word "comprise" and its derivatives such as "comprises" and "comprising" should be interpreted in an open and inclusive sense, i.e., "including but not limited to." The singular forms "a," "an," "at least one," and "the" include plural referents unless the context clearly dictates otherwise. The term "and / or," when used between the last two elements of a list of elements, means any one or more of the listed elements. The term "or" is generally used in its broadest sense, i.e., to mean "and / or" unless the context clearly dictates otherwise. When used to describe a dimensional range, the phrase "between X and Y" represents a range that includes X and Y. As used herein, "apparatus" may refer to any individual device, a collection of devices, a portion of a device, or a collection of portions of a device.

[0012] 1 illustrates a system 100 according to one implementation that can measure and correct optical aberrations in a charged particle beam microscope system while the charged particle beam microscope system is operating in a transmission imaging mode. The system 100 is illustrated in the context of an exemplary charged particle beam microscope system 104. However, the system 100 is not limited to the particular configuration of the charged particle beam microscope system 104 shown in FIG. 1 and can be used with any charged particle beam microscope system operating in a transmission imaging mode.

[0013] In the illustrated example, the charged particle beam microscope system 104 includes a charged particle beam microscope column 120 (hereinafter microscope column 120) enclosed within a vacuum chamber 106. The microscope column 120 defines an optical axis 116 of the charged particle beam microscope system 104.

[0014] The sample 124 to be interrogated can be positioned along the optical axis 116. The sample 124 can be supported by a sample holder 140, and in some examples can have the ability to translate, rotate, and / or tilt the sample 124 relative to the optical axis 116. The sample holder 140 can allow different regions of the sample 124 to be positioned relative to the optical axis 116 for interrogation.

[0015] At the top of the microscope column 120 is a charged particle source 108 (e.g., a thermionic electron source, a Schottky emission source, a field emission source, etc.) that emits a charged particle beam 112 (e.g., an electron beam). The charged particle beam 112 is directed along an optical axis 116 so that it can impinge on and interact with a sample 124. The charged particle beam 112 passes through the sample 124 to create an image.

[0016] A camera 136 at the bottom of the microscope column 120 can be operated to capture images of the sample 124. The camera 136 can include, for example, a CCD (charge-coupled device) imaging sensor, a CMOS (complementary metal-oxide semiconductor) imaging sensor, or more generally, an array of photodetectors. In one example, the camera 136 can be operable in a "movie" mode to capture a series of images of the sample.

[0017] The microscope column 120 may include various charged particle beam lens systems (hereinafter lens systems) that direct the charged particle beam 112 along an optical axis 116 and configure the charged particle beam 112 to provide consistent illumination over a desired field of view on the sample 124. In some examples, the various lens systems may also project an image produced by the interaction of the charged particle beam 112 with the sample 124 into the field of view of the camera 136.

[0018] In some examples, the various lens systems may include a condenser lens system 122, an objective lens system 126, and a projection lens system 130, which may have any suitable configuration for modifying the charged particle beam 112 to provide a desired field of view. The condenser lens system 122 may be configured to focus the charged particle beam 112 into a substantially parallel beam. The objective lens system 126 may be configured to focus an image formed by the transmission of the charged particle beam 112 through the sample 124. The projection lens system 130 may be configured to project the image coming from the objective lens system 126 into a field of view of the camera 136.

[0019] In some examples, the condenser lens system 122 can include condenser lenses 122a, 122b and other components such as a condenser stigmator 122c (i.e., a device that corrects axial astigmatism) and a condenser aperture 122d.

[0020] In some examples, the objective lens system 126 can include an objective lens 126a and other components, such as a beam deflector 126b (which can be used to center the beam), an objective stigmator 126c (i.e., a device for correcting axial astigmatism), and an objective aperture 126d. In the illustrated example, the sample 124 is positioned above the objective lens 126a (in other examples, the sample 124 can be inserted into the objective lens 126a).

[0021] In some examples, projector lens system 130 can include, for example, intermediate lens 130a (e.g., for magnifying the image) and projector lenses 130b, 130c. Projector lens system 130 can include other components such as a stigmator, an image deflector, and an aperture.

[0022] The components of the lens system may be electro-optical components that are adjustable by varying the current applied to the components. For example, the various lenses of the lens systems 122, 126, 130 may be electromagnetic lenses. Electromagnetic lenses modify the charged particle beam by a magnetic field generated by a solenoid. By varying the current to the solenoid, the magnetic field can be changed, which correspondingly changes the focal length and magnification of the lens. Other components of the lens systems 122, 126, 130, such as the stigmators and beam deflectors, may be electromagnetic devices that can be adjusted in the same manner as described for the electromagnetic lenses. The ability to tune the components of the lens systems 122, 126, 130 by varying the current supplied to the components may allow aberration corrections to be applied to the microscope column 120 on the fly (e.g., while the charged particle beam microscope system 104 is operating in a transmission imaging mode).

[0023] The lens systems 122, 126, 130 may include other optical components or different configurations of optical components not shown in Figure 1. The microscope column 120 may include other optical components near the output of the charged particle source 108. For example, the microscope column 120 may include an accelerator stack that can be used to increase the acceleration of the charged particle beam 112 (e.g., prior to focusing the charged particle beam 112).

[0024] The system 100 includes a pattern generator 144 that can generate a variety of patterns. The pattern generator 144 can be, for example, a programmable function generator. In some examples, the pattern generator 144 can be configured to generate a periodic pattern. In one example, the periodic pattern can be a Lissajous figure. The Lissajous figure is given by the following parametric equation:

[0025]

number

[0026] A Lissajous figure can be as simple as a circle (e.g., the parametric equations (1) and (2) give a circle when a / b=1, A=B, and d=p / 2 radians), or it can be more complex (e.g., the ratio a / b does not equal 1).

[0027] The system 100 includes a beam deflector 134 inserted in the microscope column 120. The beam deflector 134 can be controlled according to a pattern to tilt the charged particle beam 112 in the X and Y directions over time. The beam deflector 134 can apply a tilt motion to the charged particle beam 112 before it reaches the sample 124 (e.g., before focusing the charged particle beam 112). The beam deflector 134 can be an electro-optical deflector (e.g., an electrostatic or magnetic beam deflector) that allows changing the tilt angle applied to the charged particle beam 112 by changing the current to the beam deflector.

[0028] The beam deflector 134, operating according to a pattern, applies a time sequence of beam tilts to the charged particle beam 112. Each beam tilt creates an image. Thus, the time sequence of beam tilts results in a sequence of images. Image shifts between successive (or adjacent) images in the sequence can indicate optical aberrations in the microscope system.

[0029] The system 100 may include a frame grabber 148 in communication with the camera 136. The frame grabber 148 may, for example, form an interface between the camera 136 and a computing system 154 (e.g., a server). The frame grabber 148 is an electronic device that may capture individual image frames from the stream of image data produced by the camera 136.

[0030] The system 100 may include an image shift tracker 152 in communication with the frame grabber 148. The image shift tracker 152 may receive image frames from the frame grabber 148 and measure (or calculate) the image shift between successive image frames over time. The image shift tracker 152 may use any suitable image processing technique to measure the image shift. One exemplary technique is cross-correlation. In one example, the image shift tracker 152 may be executed by a processor (e.g., a GPU, a CPU, or an FPGA) of the computing device 154.

[0031] The system 100 can include an aberration calculator 156 that determines values ​​of optical aberrations in the charged particle beam microscope system based on the image shift measured by the image shift tracker 152. Examples of optical aberrations that can be found and measured in a charged particle beam microscope are defocus, 2-fold astigmatism, 3-fold astigmatism, 4-fold astigmatism, 5-fold astigmatism, 6-fold astigmatism, axial coma, 5th order axial coma, spherical aberration, 6th order spherical aberration, star aberration, 6th order star aberration, 3-lobe aberration, and rosette aberration. The aberration calculator 156 can be configured to determine values ​​of any combination of these optical aberrations as well as other optical aberrations not specifically mentioned.

[0032] In one example, the aberration calculator 156 can receive a stream of image shifts from the image shift tracker 152 and calculate the optical aberrations in the microscope column 120 based on the image shifts. In one example, the aberration calculator 156 can estimate the optical aberrations by fitting the image shifts to an optical aberration model that provides a relationship between the image shift, the beam tilt, and the optical aberrations in the microscope column 120. In some examples, the optical aberration model can be extended with one or more auxiliary parameters that are also fitted. The auxiliary parameters can capture the variance of the measurements. For example, the duration between generating a tilt position and actually tilting the beam at that tilt position can vary such that there is an unknown time delay between the applied beam tilt and the sequence of images captured from the application of the beam tilt. In some examples, the beam tilt control timing can be decoupled from or not synchronized with the imaging sensor. In such a case, one such auxiliary parameter can be an unknown time delay or time synchronization between the pattern controlling the beam tilt and the images captured by the camera 136. Another example of an auxiliary parameter can include sample drift. With a periodic tilt pattern, the phase of the periodic tilt pattern can be related to the phase of the detected pattern to provide timing information. In some examples, a portion of a tilt pattern period between one and two periods, or more than two periods, can be sufficient to provide the time used to synchronize clock timing between beam control and image detection. A variety of tilt patterns can be used, including non-periodic motion.

[0033] In another example, the aberration calculator 156 can determine the value of the optical aberration based on the optimization of a model that provides a relationship between a series of images and the optical aberrations in the microscope column 120. For example, an image acquired at one tilt angle can be transformed into an image acquired at another tilt angle. The transformation can be parameterized by variables related to the optical aberrations. These parameters correspond to a model. An optimization routine can find the optical parameters that can maximize the similarity of the transformed images, which can be used to determine the value of the optical aberrations. This procedure can be extended to multiple images.

[0034] In another example, the aberration calculator 156 can determine the value of the optical aberration from an inference of a neural network trained to predict the optical aberration from image information obtained by applying a sequence of beam tilts to a charged particle beam in a pattern. The trained neural network can receive as input a sequence of images and / or information derived from the sequence of images (e.g., image power spectrum, image shift, etc.) and generate an inference that can include predictions and other information such as the reliability of the predictions. In this example, the aberration calculator 156 can receive image data directly from the camera 136 or image frames from the frame grabber 148, pre-process the image data, and provide the pre-processed image data to the neural network. In some cases, the inference can be a service provided via the cloud.

[0035] The system 100 can include an aberration corrector 168 communicatively coupled to the aberration calculator 156. The aberration corrector 168 can determine a correction to apply to a component of the microscope column 120 (e.g., a component of a lens system) based on the optical aberrations determined by the aberration calculator 156. For example, if the component is an electro-optical component, the aberration corrector 168 can vary a current applied to the component to change how the component modifies the charged particle beam 112. In some examples, the aberration corrector 168 can apply a correction to the component of the microscope column 120 based on a predetermined criteria (e.g., if the optical aberration exceeds a predetermined threshold or if the reliability of the estimated aberration value exceeds a predetermined threshold (e.g., if the absolute value of the estimated aberration value is less than a confidence interval for that optical aberration)).

[0036] In some examples, the aberration corrector 168 can determine adjustments to make to the pattern generated by the pattern generator 144 due to beam tilt. For example, the aberration calculator 156 can provide optical aberrations with a certain level of confidence. If the level of confidence is low, the aberration corrector 168 can determine an optimal pattern to use in subsequent beam tilts to improve the reliability of the measurement. The aberration corrector 168 can further determine parameters of the optimal pattern (such as fundamental frequency and amplitude) and provide this information to the pattern generator 144.

[0037] In some examples, the aberration calculator 156 and the aberration corrector 168 may be programs or scripts running on a computing system 160 communicatively coupled to the charged particle beam microscope system 104. The aberration corrector 168 may be in communication with the aberration calculator 156 and the pattern generator 144. In some examples, the computing system 160 may be coupled to a display 164 (e.g., a console of the charged particle beam microscope system 104). In some examples, the computing system 160 may present current aberration values ​​and aberration trends on the display 164.

[0038] The image shift tracker 152, the aberration calculator 156, and the aberration corrector 168 can be implemented in software and / or hardware. The software components of the image shift tracker 152, the aberration calculator 156, and the aberration corrector 168 can be stored in one or more computer readable storage media or devices and executed by one or more processors (e.g., processors in computing systems 160 or 154). Any of the computer readable media herein can be non-transitory (e.g., non-volatile memory such as volatile memory, such as DRAM or SRAM, magnetic storage, optical storage, etc.) and / or tangible.

[0039] 2A is a flow diagram illustrating a method 200 of measuring and correcting optical aberrations in a charged particle beam microscope system (e.g., the charged particle beam microscope system 104 illustrated in FIG. 1) operating in a transmission imaging mode. Although the operations are illustrated in FIG. 2A (and in other of the flow diagrams discussed therein) once each and in a particular order, the operations may be suitably reordered and / or repeated as desired (e.g., different operations performed may be performed in parallel when suitable).

[0040] At 210, the method includes operating a charged particle beam microscope system in a transmission imaging mode. Operation 210 can include generating a charged particle beam (e.g., an electron beam) using a charged particle source at the top of a microscope column and directing the charged particle beam to a sample using optical components in the microscope column. The charged particle beam interacts with the sample to produce an image. Operation 210 can include projecting the image into a field of view of a camera at the bottom of the microscope column.

[0041] At 220, as described in operation 210, the method can include operating the charged particle beam microscope system in a transmission imaging mode to apply a pattern of time-sequenced beam tilts to the charged particle beam emerging from the charged particle source. Operation 220 can include generating the pattern. Operation 220 can include applying the time-sequenced beam tilts by traversing the pattern. At each point on the traversal path, the x,y coordinates of the point determine the amount of tilt applied to the charged particle beam in the X and Y directions. The amount of tilt can be used to control a beam deflector inserted in the microscope column to tilt the beam. In some examples, the pattern can be a periodic pattern. In some examples, the periodic pattern can be a Lissajous figure (or Lissajous curve).

[0042] At 225, while applying beam tilts to the charged particle beam as described in operation 220, the method may include operating the camera in a video mode to capture images of the sample. For every beam tilt, the camera may obtain a complete image of the sample. Thus, the camera may capture a series of images corresponding to applying a time series of beam tilts to the charged particle beam. The time between the application of the beam tilt and the actual tilt of the beam may vary such that there is an unknown time delay between the beam tilt pattern and the captured series of images. In some examples, the optical aberration model may incorporate the unknown time delay as an auxiliary variable to be fitted. Other auxiliary variables, such as the beam tilt pattern or sample drift, may also be incorporated into the optical aberration model.

[0043] At 230, the method can include estimating values ​​of optical aberrations in the microscope column from the images captured in operation 225. In one example, the optical aberrations can be estimated from an optical aberration model that relates image shifts to beam tilt and optical aberrations of the microscope column. In another example, the optical aberrations can be obtained by optimizing a model that provides a relationship between the series of images and the optical aberrations in the microscope column. In another example, the optical aberrations can be obtained as an inference of a trained neural network. The inference can include predictions of the neural network as well as other information such as the reliability of the predictions.

[0044] FIG. 2B illustrates one example of operation 230 of FIG. 2A. In the example illustrated in FIG. 2B, at 240, the method includes determining image shifts from the series of images captured in operation 225. Operation 240 can include receiving image data from a camera and extracting a series of images (or image frames) from the image data. Each image corresponds to an image of the sample at a given beam tilt. Any shifts between successive (or adjacent) images depend on the optical aberrations of the charged particle beam microscope. Operation 240 can include determining these image shifts using any suitable image processing technique, such as cross-correlation.

[0045] For illustrative purposes, FIG. 3A shows a beam tilt pattern 300 that can be applied to a charged particle beam, and FIGS. 3B-E show expected image shift patterns 302, 304, 306, 308 produced from applying the beam tilt pattern 300 due to various types of microscope aberrations (e.g., defocus, astigmatism, coma, and spherical aberration). The beam tilt pattern 300 is a Lissajous pattern (a=3, b=2, where a and b are indicated in equations (1) and (2)). FIGS. 3B-E show the motion of the image of the sample over time as the beam tilt pattern is applied. If the microscope has no optical aberrations, the beam tilt pattern does not induce a shift in the images of the sample collected over time. However, if the microscope has aberrations, the pattern formed by the motion of the sample will be different (e.g., scaled and / or distorted) from the applied beam tilt pattern.

[0046] The image shift pattern 302 shown in Figure 3B corresponds to an example where the optical aberration in the microscope is defocus. The image shift pattern 302 and the applied beam tilt pattern 300 are linearly dependent (i.e., the image shift pattern 302 is scaled relative to the applied beam tilt pattern 300, and the amount of defocus is a scaling factor between the diagrams shown in Figures 3A and 3B).

[0047] The image shift pattern 304 shown in Figure 3C corresponds to the example where the optical aberrations of the microscope are defocus and astigmatism: the image shift pattern 304 compared to the applied beam tilt pattern 300 is distorted in one direction (by astigmatism) and scaled (by defocus).

[0048] The image shift pattern 306 in Figure 3D corresponds to an example where the optical aberrations of the microscope are defocus, astigmatism, and coma. The image shift pattern 306 compared to the applied beam tilt pattern 300 is asymmetrically distorted (by coma), deformed in one direction (by astigmatism), and scaled (by defocus).

[0049] The image shift pattern 308 shown in Figure 3E corresponds to an example where the optical aberrations in the microscope are defocus, astigmatism, coma, and spherical aberration. The image shift pattern 308 compared to the applied beam tilt pattern 300 is radially distorted (by spherical aberration), asymmetrically distorted (by coma), distorted in one direction (by astigmatism), and scaled (by defocus).

[0050] FIG. 4A illustrates another example of a beam tilt pattern 400 that can be applied to a charged particle beam, and FIGS. 4B-4D illustrate expected image shift patterns 402, 404, 406 produced from applying the beam tilt pattern 400 due to various types of microscope aberrations (e.g., defocus, astigmatism, and coma). The beam tilt pattern 400 is a circular pattern (a special case of a Lissajous figure). FIGS. 4B-4D show the motion of the image of the sample versus time when the beam tilt pattern is applied. If the microscope has no optical aberrations, the beam tilt pattern does not induce a shift in the image of the sample collected over time. However, if the microscope has aberrations, the pattern formed by the motion of the sample will be different (e.g., scaled and / or distorted) from the applied beam tilt pattern.

[0051] Image shift pattern 402 in FIG. 4B corresponds to an example where the optical aberration of the microscope is defocus (image shift pattern 402 is scaled compared to the applied beam tilt pattern 400, and the amount of defocus is the scaling factor between the diagrams shown in FIG. 4A and FIG. 4B). Image shift pattern 404 in FIG. 4C corresponds to an example where the optical aberration in the microscope is defocus and astigmatism (image shift pattern 404 is scaled and distorted in one direction). Image shift pattern 406 in FIG. 4D corresponds to an example where the optical aberration of the microscope is defocus, astigmatism, and coma (image shift pattern 406 is scaled, distorted in one direction, and asymmetrically distorted).

[0052] A circular pattern, such as the beam tilt pattern 400 shown in Figure 4A, can effectively capture low-order aberrations (e.g., defocus), but may be less sensitive to higher-order aberrations, for which a more complex Lissajous figure, such as the beam tilt pattern 300 shown in Figure 3A, may be more suitable.

[0053] 2B, at 245, the method includes estimating a value of the selected optical aberration from the image shift obtained in operation 240. The aberration value may be estimated by fitting the image shift to an optical aberration model of the charged particle beam microscope system.

[0054] The following Equation (3) is an example of an optical aberration model. For illustrative purposes, the model in Equation (3) can obtain values ​​for defocus, astigmatism (astigX, astigY), coma (comaX, comaY), three-fold astigmatism (threefoldX, threefoldY), spherical aberration (spherical), star aberration (starX, starY), and four-fold astigmatism (fourfoldX, fourfoldY).

[0055]

number

[0056] The vector on the right hand side of the equation contains the values ​​of the optical aberrations for a given state of the optical system of the microscope system at a certain moment. When the beam tilt is adjusted by (tx,ty) radians, the induced image shift is given by (shiftX,shiftY), which can be measured from an image captured while applying the beam tilt (e.g., in operation 240). The measured shiftX, shiftY and the known tx,ty can be fitted to the model of equation (3) to obtain an estimate of the optical aberrations. The parameters tx,ty are time dependent, allowing the model to take into account the unknown time delay between the applied beam tilt pattern and the induced image shift pattern for the fitting. Variations of the optical aberration model can incorporate other auxiliary parameters such as the beam tilt pattern and / or sample drift.

[0057] Returning to FIG. 2A, at 250, the method includes determining a correction to apply to optical components within the microscope column based on the aberration values ​​estimated in act 230 (or act 245 in FIG. 2B).

[0058] At 260, the method may include applying the corrections determined in operation 250 to one or more of the optical components in the microscope column based on a predetermined criteria. For example, if an estimate of one or more of the selected aberrations is above a predetermined threshold, the corrections may be applied. Alternatively, if an estimate of the selected aberrations is below a predetermined threshold, the corrections may not be applied. However, the corrections may be stored and used during post-processing of the captured image of the sample.

[0059] At 270, the method may include adjusting the pattern used for beam tilting in operation 220 based on a predetermined factor. For example, the pattern may be adjusted if, for example, the accuracy of an estimate of one or more selected optical aberrations is below a predetermined threshold, if the quality of the images in the series of images is below a predetermined threshold, and / or if there is a disturbance in capturing the series of images (e.g., mechanical vibrations). The pattern may be adjusted by varying parameters of the pattern (e.g., the fundamental frequency and / or amplitude of the pattern may be changed), or by selecting a new pattern that is optimal for the microscope and sample configuration.

[0060] Operations 220-270 can be repeated until the end of the planned acquisition, or until the aberrations are measured with the desired accuracy, or until the aberrations are matched to the desired values ​​with the desired accuracy. The aberration measurements over time can be displayed and monitored on a screen.

[0061] FIG. 5A illustrates an exemplary method 500 of operating a charged particle beam microscope system (e.g., system 100 illustrated in FIG. 1) with aberration monitoring and correction. Method 500 is optimized to accurately and precisely measure rapidly changing aberrations. As noted above with respect to FIG. 2A and others in the accompanying flow diagrams, although the operations are illustrated in FIG. 5A in a particular order, each once, the operations may be appropriately reordered and / or repeated as desired (e.g., different operations performed may be performed in parallel when suitable). For example, method 500 may include performing in parallel any suitable combination of the operations presented therein.

[0062] At 510, with the sample positioned along the optical axis of the charged particle beam microscope system, the method 500 includes navigating to a region of interest on the sample.

[0063] At 520, the method 500 includes measuring values ​​of aberrations in the charged particle beam microscope system. In one example, the measurements can be performed according to acts 210-230 of FIG. 2A.

[0064] At 530, the method 500 includes determining a correction for a rapidly varying aberration (eg, defocus) in the charged particle beam microscope system.

[0065] At 540, the method 500 includes monitoring changes in the values ​​of other optical aberrations (e.g., optical aberrations that do not change rapidly, such as 2-fold astigmatism, 3-fold astigmatism, 4-fold astigmatism, 5-fold astigmatism, 6-fold astigmatism, axial coma, 5th order axial coma, spherical aberration, 6th order spherical aberration, star aberration, 6th order star aberration, 3-lobe aberration, or rosette aberration). The changes in the optical aberrations can be presented on a display, for example in the form of a trend graph.

[0066] At 550, the method 500 may include determining corrections for other aberrations having values ​​above a predetermined threshold.

[0067] At 560, the method 500 includes applying any corrections determined in operations 530 and 540 to an optical column (or microscope column) of the charged particle beam microscope system.

[0068] At 570, the method 500 includes acquiring an image of the sample with the corrected charged particle beam microscope system. After acquiring the image, the method 500 may return to operation 510 to process another area of ​​the sample.

[0069] Some optical aberrations (e.g., defocus) may be identified using a relatively small amount of data, while other optical aberrations (e.g., coma and astigmatism) may require a relatively large amount of data to be properly identified. Instead of waiting to identify all optical aberrations before determining and applying optical corrections to the charged particle beam microscope system, optical corrections can be more quickly determined and applied to those optical aberrations that can be identified using a relatively small amount of data.

[0070] 5B illustrates an exemplary method 580 of operating a charged particle beam microscope system (e.g., system 100 illustrated in FIG. 1) that monitors and corrects aberrations based on the principle of applying optical corrections at different frequencies. As with FIG. 2A and others in the accompanying flow diagrams, although the operations are illustrated in FIG. 5B in a particular order, once each, the operations may be suitably reordered and / or repeated as desired (e.g., different operations performed may be performed in parallel, when suitable). For example, method 580 may include performing in parallel any suitable combination of the operations presented therein.

[0071] At 581, a first set of one or more optical aberrations and a second set of one or more optical aberrations are determined. The first set of one or more optical aberrations is different from the second set of one or more optical aberrations (e.g., one or more optical aberrations in the first set are different from one or more optical aberrations in the second set). In one example, the difference between the first set and the second set can be based on the amount of data required to properly identify the optical aberrations and / or the drift rate in the microscope system (i.e., the rate at which the optical aberrations change their value significantly). In one example, the first set can include one or more optical aberrations that can be identified using a relatively small amount of data (e.g., defocus). In one example, the second set can include one or more optical aberrations that require a relatively large amount of data to be properly identified (e.g., coma and astigmatism).

[0072] At 582, the method 580 includes navigating to a region of interest on a sample disposed along an optical axis of a charged particle beam microscope system.

[0073] At 583, the method 580 includes acquiring an image of the sample while operating the charged particle beam microscope system. The image may be acquired, for example, as described in acts 210-225 of FIG. 2A.

[0074] At 584, the method 580 includes determining a first optical aberration output set for the charged particle beam microscope system. The first optical aberration output set includes a first set of values ​​of one or more optical aberrations in the charged particle beam microscope system. The first input set for determining the first optical aberration output set includes one or more of the images acquired in operation 583. The first set of values ​​of the one or more optical aberrations can be determined, for example, using an aberration calculator (156 in FIG. 1 ) and the first input set. In some examples, the first optical aberration output set can further include one or more values ​​of a second set of one or more optical aberrations in the charged particle beam microscope system. The second set of values ​​can be determined, for example, using an aberration calculator and the first input set. The first and second set of values ​​can be determined within the same aberration calculator (e.g., using the same optical aberration model, model optimization, or trained neural network). The values ​​within the first optical aberration output set can include estimates of the optical aberrations and the reliability of the estimate.

[0075] At 586, the method 580 includes applying corrections (or adjustments) to the charged particle beam microscope system at the first frequency to mitigate the first set of one or more optical aberrations. The corrections to apply to the first set of one or more optical aberrations are determined based on values ​​corresponding to the first set of one or more optical aberrations in the first optical aberration output set determined in operation 584. In one example, the corrections may be applied by varying currents supplied to one or more electro-optical components of the charged particle beam microscope system.

[0076] At 588, the method 580 includes determining a second optical aberration output set for the charged particle beam microscope system. The second optical aberration output set includes values ​​of a second set of one or more optical aberrations in the charged particle microscope system. The second input set for determining the second optical aberration output set may include one or more of the images acquired in operation 583 or may include some values ​​from the first optical aberration output set (e.g., values ​​determined for one or more of the second set of one or more optical aberrations in operation 584). In one example, the values ​​of the second set of one or more optical aberrations may be determined using an aberration calculator (156 in FIG. 1 ) and a second input set. In some examples, values ​​determined in act 584 for one or more of the second set of one or more optical aberrations may be accumulated over time and used to determine values ​​for the second optical aberration output set (e.g., values ​​determined over time for the optical aberrations in the second set in act 584 may be averaged to determine corresponding values ​​for the optical aberrations in the second optical aberration output set). Act 588 may run at a slower rate compared to act 584. Thus, one or more runs of determining the first optical aberration output set may be completed before a single run of determining the second optical aberration output set is completed (allowing for the aforementioned accumulation of data from act 584 for determining values ​​for the second optical aberration output set).

[0077] At 590, the method 580 includes applying corrections (or adjustments) to the charged particle beam microscope system at a second frequency to mitigate a second set of one or more optical aberrations whose values ​​were determined in operation 588. In one example, the second frequency in operation 590 is different from the first frequency in operation 586. For example, the second frequency is lower than the first frequency such that the corrections in operation 586 are applied more frequently than the corrections in operation 590. The second frequency may be selected based on the time required for each iteration of operation 588 to determine a sufficiently accurately set second optical aberration. In one example, the corrections may be applied by varying a current supplied to one or more electro-optical components of the charged particle beam microscope system.

[0078] In 591, the method 580 may include determining whether the value determined in either of operations 584 and 588 exceeds a predetermined threshold. If the value exceeds the predetermined threshold, the method may return to operation 583 to acquire more images of the sample with the correction applied to the charged particle beam microscope system.

[0079] At 592, method 580 may include acquiring an image of the sample with the corrected charged particle beam microscope system. In some cases, the image may be acquired when it is determined that the aberration values ​​determined in acts 584 and 588 do not exceed a predetermined threshold. After acquiring the image, method 580 may return to act 582 to process another area of ​​the sample or the same area of ​​the sample. In other examples, act 582 may be performed in parallel with other acts in method 580 (i.e., determining the aberration values ​​and making corrections to the microscope system may be performed while the sample is moving relative to the microscope system).

[0080] Figure 6 illustrates an exemplary method 600 for acquiring images of a sample using a charged particle beam microscope system with aberration monitoring and correction. As with Figure 2A and others in the accompanying flow diagrams, although the operations are illustrated in Figure 6 once each and in a particular order, the operations may be suitably reordered and / or repeated as desired (e.g., different operations performed may be performed in parallel when suitable). For example, method 600 may include performing in parallel any suitable combination of the operations presented therein.

[0081] At 610, with the sample positioned along the optical axis of the charged particle beam microscope system, the method 600 includes navigating to a region of interest on the sample.

[0082] At 620, the method 600 includes measuring values ​​of aberrations in a charged particle beam microscope system according to acts 210-230 of FIG. 2A.

[0083] At 630, the method 600 includes monitoring the change in the optical aberration over time. The change in the optical aberration can be presented on a display, for example in the form of a trend graph. In an alternative implementation of the method, the monitoring of the change in the optical aberration can be omitted.

[0084] At 640, the method 600 includes determining whether any of the aberration values ​​exceed a predetermined threshold. If any of the aberration values ​​exceed a predetermined threshold, the method 600 continues at operation 650. If not, the method 600 continues at operation 660.

[0085] At 650, method 600 includes determining a correction for the aberrations that exceed a predetermined threshold and applying the correction to the optical column of the charged particle beam microscope system. After applying the correction, method 600 returns to operation 620 to repeat the measurement of the aberration values. This can continue until no aberration values ​​exceed the predetermined threshold.

[0086] At 660, the method 600 includes acquiring an image of the sample with the corrected charged particle beam microscope system. After acquiring the image, the method 600 may return to operation 610 to process another area of ​​the sample.

[0087] Operations in the exemplary methods 500, 580, 600 may be controlled by computer programs that may be stored on one or more computer-readable storage media and executed by one or more processors of a computing system. The computer-readable storage media may be non-transitory (e.g., volatile memory such as DRAM or SRAM, non-volatile memory such as magnetic storage, optical storage, etc.) and / or tangible.

[0088] 7 is a block diagram of a generalized exemplary computing system (or computing environment) that may be used in the methods described herein. In some examples, the methods may use a single computing system. In other examples, the methods may use multiple computing systems that may communicate over a network.

[0089] Referring to Figure 7, a computing system 700 includes one or more processing units 710, 715, and memory 720, 725. In Figure 7, this basic configuration 730 is included within a dashed line. The processing units 710, 715 execute computer-executable instructions. A processing unit may be a general-purpose central processing unit (CPU), an application-specific integrated circuit (IC), or a combination of both. The tangible memory 720, 725 may be a processor in a general-purpose integrated circuit (ASIC), a processor in a 3D image processing system, or any other type of processor. In a multi-processing system, multiple processing units execute computer-executable instructions to increase processing power. For example, FIG. 7 shows a central processing unit 710 as well as a graphics processing unit or co-processing unit 715. The tangible memory 720, 725 may be volatile memory (e.g., registers, cache, RAM), non-volatile memory (e.g., ROM, EEPROM, flash memory, etc.) accessible by the processing units, or some combination of the two. The memory 720, 725 stores software 780 that implements one or more innovations described herein in the form of computer-executable instructions suitable for execution by the processing units. The software 780 may include, for example, one or more of the logic of the image shift tracker 152, the aberration calculator 156, the aberration corrector 168, or the methods 500, 580, 600.

[0090] The computing system may have additional features. For example, computing system 700 may include a storage device 740, one or more input devices 750, one or more output devices 760, and one or more communication connections 770. An interconnection mechanism (not shown), such as a bus, controller, or network, interconnects the components of computing system 700. Typically, operating system software (not shown) provides an operating environment for other software executing within computing system 700 and coordinates the activities of the components of computing system 700.

[0091] Tangible storage 740 may be removable or non-removable and includes magnetic disks, magnetic tapes or cassettes, CD-ROMs, DVDs, or any other medium that can be used to non-transiently store information and that can be accessed within computing system 700. Storage 740 stores instructions for software 780 that implements one or more of the innovations described herein.

[0092] The input device 750 may be a touch input device such as a keyboard, a mouse, a pen, or a trackball, a voice input device, a scanning device, or another device that provides input to the computing system 700. The output device 760 may be a display, a printer, speakers, a CD writer, or another device that provides output from the computing system 700.

[0093] The communications connection(s) 770 enable communication over a communications medium to another computing entity. The communications medium conveys information such as computer-executable instructions, audio or video input or output, or other data in a modulated data signal. A modulated data signal is a signal that has one or more characteristics set or changed in such a manner as to encode information in the signal. By way of example, and not limitation, communications media may use electrical, optical, RF, or other carrier waves.

[0094] In view of the above implementations of the disclosed subject matter, the present application discloses additional examples, which are listed below. It should be noted that a feature of a single section, or a combination of two or more features of that section, optionally in combination with one or more features of one or more additional examples, is also a further example included in the disclosure of the present application.

[0095] Example 1 is a method of directing a charged particle beam to a sample, including operating a charged particle beam microscope system in a transmission imaging mode to apply a time sequence of patterns of beam tilts to the charged particle beam to produce a series of images, where at least some of the images in the series of images are captured while the charged particle beam transitions between one beam tilt in the time sequence and successively adjacent beam tilts in the time sequence, the pattern being configured to induce image changes between images in the series of images that are indicative of optical aberrations in the charged particle beam microscope system. The method includes capturing the series of images.

[0096] Example 2 includes the subject matter of example 1, further specifying that the pattern is a periodic pattern.

[0097] Example 3 includes the subject matter of example 2, and further specifies that the periodic pattern is a Lissajous figure.

[0098] Example 4 includes the subject matter of any of examples 1-3, further including generating a set of one or more optical aberration values ​​of the charged particle beam microscope system from the series of images.

[0099] Example 5 includes the subject matter of example 4, wherein the image change includes an induced image shift, and generating a set of one or more optical aberration values ​​of the charged particle beam microscope system from the series of images includes measuring the induced image shift between successive images in the series of images.

[0100] Example 6 includes the subject matter of example 5, and further includes generating a set of one or more optical aberration values ​​of the charged particle beam microscope system from the series of images includes obtaining inferences from a neural network model trained to identify the set of optical aberrations from input data derived from the series of images.

[0101] Example 7 includes the subject matter of example 5, and further includes generating a set of one or more optical aberration values ​​of the charged particle beam microscope system from the series of images includes fitting the induced image shift to an optical aberration model that describes a relationship between the induced image shift, beam tilt, and the set of optical aberrations.

[0102] Example 8 includes the subject matter of Example 7 and further specifies that the optical aberration model incorporates an unknown time delay between the pattern and the series of images.

[0103] Example 9 includes the subject matter of any of Examples 4-8, wherein generating a set of one or more optical aberration values ​​of the charged particle beam microscope system from the series of images includes estimating values ​​of one or more of: defocus, 2-fold astigmatism, 3-fold astigmatism, 4-fold astigmatism, 5-fold astigmatism, 6-fold astigmatism, axial coma, 5th order axial coma, spherical aberration, 6th order spherical aberration, star aberration, 6th order star aberration, 3-lobe aberration, and rosette aberration.

[0104] Example 10 includes the subject matter of any one of examples 4-9, further including determining optical corrections for the charged particle beam microscope system based on the set of one or more optical aberration values.

[0105] Example 11 includes the subject matter of example 10, further including applying the optical correction for the charged particle beam microscope system to one or more optical components of the charged particle beam microscope system.

[0106] Example 12 includes the subject matter of example 10, and further includes adjusting the pattern based on one or more of: accuracy of the set of one or more optical aberration values, quality of an image in the series of images, and disturbances in capturing the series of images.

[0107] Example 13 includes the subject matter of any one of examples 4-12, further including monitoring a change in the set of one or more optical aberrations over time.

[0108] Example 14 is a method that includes receiving, by one or more processors, a series of images captured while applying a periodic pattern of time-sequenced beam tilts to a charged particle beam transmitted along an optical axis of an optical column of a charged particle beam microscope system and through a sample positioned along the optical axis, the method including estimating, by the one or more processors, values ​​of one or more optical aberrations produced by the optical column from the series of images.

[0109] Example 15 includes the subject matter of example 14, further including determining one or more optical corrections for one or more optical components in the optical column; and selectively applying the one or more optical corrections to the one or more optical components.

[0110] Example 16 includes the subject matter of any one of examples 14-15, further specifying that the periodic pattern is a Lissajous figure pattern.

[0111] Example 17 is a system including a charged particle source that emits a charged particle beam, the system comprising: an optical column having one or more optical components configured to define an optical axis and form the charged particle beam within a field of view of a sample positioned along the optical axis; a pattern generator configured to generate a pattern; one or more beam deflectors positioned along the optical axis and controllable by the pattern to apply a time series of beam tilts to the charged particle beam; an imaging sensor positioned along the optical axis and capturing images of the sample; one or more processors; and one or more computer-readable storage media storing instructions that, when executed by the one or more processors, cause the one or more processors to receive a series of images captured by the imaging sensor during application of the time series of beam tilts and cause the one or more processors to generate a set of one or more optical aberration values ​​of the optical system from the series of images, where at least some of the images in the series of images are captured during movement of the charged particle beam between successive beam tilts in the time series of beam tilts.

[0112] Example 18 includes the subject matter of Example 17, and further specifies that the one or more computer-readable storage media further store instructions that, when executed by the one or more computing devices, cause the one or more computing devices to determine an induced image shift in the series of images and determine values ​​of one or more optical aberrations from an applied time sequence of the induced image shift and beam tilt corresponding to the induced image shift.

[0113] Example 19 includes the subject matter of example 17, and further specifies that the one or more computer-readable storage media further store instructions that, when executed by the one or more computing devices, cause the one or more computing devices to determine one or more optical corrections for the one or more optical components based on the set of one or more optical aberration values.

[0114] Example 20 includes the subject matter of any one of Examples 17-19, and further specifies that the one or more computer-readable storage media further store instructions that, when executed by the one or more computing devices, cause the one or more computing devices to determine an adjustment to the pattern generated by the pattern generator based on a confidence level of the determined values ​​for the one or more optical aberrations.

[0115] Example 21 includes the subject matter of any one of examples 17-20, further specifying that the pattern generator is configured to generate a periodic pattern.

[0116] Example 22 is a method of operating a charged particle beam microscope system, the method including: determining a first optical aberration output set including values ​​for a first set of one or more optical aberrations in the charged particle beam microscope system based on a first input set including one or more acquired images; applying adjustments to the charged particle beam microscope system at a first frequency based on the first optical aberration output set to reduce the first set of optical aberrations; determining values ​​of a second optical aberration output set including values ​​of a second set of one or more optical aberrations of the charged particle beam system based on a second input set including the first input set or at least a portion of the first optical aberration output, the one or more optical aberrations in the second set being different from the one or more optical aberrations in the first set; and applying adjustments to the charged particle beam microscope system at a second frequency based on the second optical aberration output set to reduce the second set of aberrations, the second frequency being less than the first frequency.

[0117] Example 23 includes the subject matter of example 22, further specifying that the first set of one or more optical aberrations includes defocus.

[0118] Example 24 includes the subject matter of any one of Examples 22-23, further specifying that the second set of one or more optical aberrations includes coma or astigmatism.

[0119] Example 25 includes the subject matter of any one of Examples 22-24, and further includes determining the first set of optical aberration outputs based on the first set of inputs including determining values ​​of at least some of the one or more optical aberrations in a second set, where at least some of the first set of optical aberration outputs included in the second input set include at least some of the values.

Claims

[Claim 1] 1. A method comprising: directing a charged particle beam at a sample, comprising operating a charged particle beam microscope system in a transmission imaging mode to apply a time sequence of beam tilts to the charged particle beam in a periodic pattern of a Lissajous figure to produce a series of images, wherein at least some of the images in the series of images are captured while the charged particle beam transitions between one beam tilt in the time sequence and successively adjacent beam tilts in the time sequence, the pattern being configured to induce image changes between images in the series of images that are indicative of optical aberrations in the charged particle beam microscope system; capturing said series of images.

Images